The present invention relates to a data reproducing apparatus, and more particularly to an apparatus for producing a synchronous signal which is synchronous with record data, when reproducing the record data with external peripheral equipment of a computer such as an optical disc unit, a magnetic disc unit, a magneto-optical disc unit or a magnetic card reader.
In an optical disc unit for example, a synchronous signal is produced using a regenerative clock signal of a VFO portion in a sector read out from an optical disc, and on the basis of the resultant synchronous signal, a PLL circuit is synchronized. In this connection, if dust or the like is stuck to the VFO portion on the optical disc, errors may occur in the synchronous signal so that the synchronous processing of the PLL circuit gets out of order in some cases. In a case where a defective portion is present in the VFO portion due to sticking of the dust or the like, in the prior art apparatus, a retry operation of reading the same portion is repeated to obtain a synchronous signal free from the errors. However, even if the retry is repeatedly performed, it is difficult to produce a synchronous signal free from errors as long as the dust thus stuck is not removed.
Incidentally, the operation of the PLL circuit which is used for such a data reproducing operation is disclosed in JP-A-1-277371 and JP-A-2-83862.